The present invention relates generally to submersible electrical motors used in liquid pumping applications. More particularly, the invention relates to improvements designed to both reduce the build-up of heat within a submersible electric motor while it is in operation, and to provide lubrication to the motor's moving parts.
Because they are adapted to fit within a narrow well bore, submersible electric motors which are used to power pumps for removing water and other liquids from underground wells normally have a relatively long length and small, cross-sectional diameter. The main body of such motors is typically formed from a tubular metal shell or casing. A rotatable drive shaft is mounted along the motor's central vertical axis; a coupling end of the drive shaft extends from the top of the motor assembly to provide motive power to a centrifugal pump mounted directly above the motor. In its operative environment, the composite pump and motor assembly lies beneath the surface of the liquid in a well. A motor of this type is disclosed in U.S. Pat. No. 3,777,194.
The motor of the present invention is of the pre-filled type, i.e., a motor where a sealed cavity in the interior of the motor containing the stator and rotor is filed with a liquid or fill solution such as mineral oil. When electric power is supplied to the stator windings of an electric motor, flow of current through the stator windings and rotor will generate heat. In combination with the motor casing, this liquid fill solution functions as a heat transfer medium to prevent excessive heat build-up within the motor interior while electrical power is applied to the motor. The liquid can also function as a lubricant for the bearing assemblies used to mount the drive shaft of the electric motor. Because both heat and friction can shorten the life of a motor, movement of the liquid fill solution within the motor cavity is desirable to aid in heat dissipation and motor lubrication.
The radial side load on the bearing adjacent to the coupling end of the motor drive shaft is generally higher than the radial load experienced by the lower end bearing. Therefore, the supply of sufficient lubricating fluid fill solution to the coupling end, i.e., the top end bearing of a submersible motor of the pre-filled type, is very critical.
As the interior of an electric motor heats up during operation, thermal expansion of the liquid fill in the oil-filled motor can occur. Some fill solution can leak out of the motor casing into the well fluid, typically around the motor's top seals where the spine or coupling end of the motor shaft extends from the top of the motor casing for connection with a pump. This leakage can eventually result in the loss of liquid fill in the upper regions of the motor, particularly in the area around the bearing assembly adjacent the upper portion of the motor drive shaft. The consequent loss of lubrication in this area may result in motor failure.
Accordingly, there is a need for a useful means to provide enhanced lubrication to the top end bearing in the types of submersible motors disclosed herein.